Study on the alignment control of liquid crystalline molecules by photoisomerization reaction

利用拉曼散射、紫外-可见和红外吸收光谱研究了4-（4 '-（甲氧基苯偶氮）苯氧基）十一烷硫醇（AZO 11）在蒸镀金膜上自组装单层膜的光致异构化反应.与AZO 11在溶液中的异构化相比，自组装AZO 11分子的反 * 顺光异构化受到限制。此外，反式至顺式异构化比顺式至反式异构化受到更多抑制，正如从顺式AZO 11的分子横截面大于反式AZO 11所预期的。在此基础上，我们提出顺式样品总体上更适合于顺反光致异构化，以控制液晶的分子取向。这确实在我们的实验中得到了验证，其中在顺式和反式AZO 11表面层上沉积了顺式液晶（4-氰基-4 '-戊基联苯：5CB）的薄膜。红外反射吸收光谱表明，5CB分子倾向于与它们的长轴垂直的反式AZO 11层的平面对齐。然而，由于部分转化为顺式异构体，这种对齐受到UV照射的影响。这一事实表明，液晶取向可以通过改变基板表面上的分子形状来控制。在吸附AZO_（11）和十六烷基硫醇（C_ H_ SH）的混合物覆盖的金膜表面上进行了类似的IR测量<16><33>。在这种情况下，AZO 11光异构化的限制被放松，并且观察到液晶取向的较大变化。红外透射和拉曼散射测量银岛膜的实用性也得到了证明。

The photoisomerization of self-assembled monolayer films of 4-(4'-(methoxyphenylazo)phenoxy)undecane- 1-thiol (AZO 11) on evaporated gold films was investigated by means of Raman scattering and UV-visible and IR absorption measurements. The trans * cis photoisomerization of the self-assembled AZO11 molecules were restricted in comparison to the isomerization of AZO11 in solutions. Moreover, the trans-to-cis isomerization was more suppressed than the cis-to-trans isomerization, as expected from the molecular cross section of cis AZO11 that is larger than trans AZO11. On this basis we propose that cis-form samples are on the whole more suitable for cis trans photoisomerization in order to control the molecular alignment of liquid crystals. This indeed was verified in our experiments where thin films of a nematic liquid crystal (4-cyano-4'-pentylbiphenyl : 5CB) were deposited on the cis and trans AZO11 surface layers. IR reflection absorption spectroscopy revealed that 5CB molecules tend to align with their long axes perpendicular to the plane of the trans AZO11 layer. However, this alignment was affected by UV irradiation due to partial conversion to cis isomer. This fact demonstrates that liquid crystal alignment can be controlled by changes in the molecular shape on the substrate surface. Similar IR measurements were performed on gold film surfaces covered with a mixture of adsorbed AZO11 and hexadecanethiol (C_<16>H_<33>SH). in this case the limitation of AZO11 photoisomerization was relaxed and a larger change in the liquid crystal alignment was observed. The utility of IR transmission and Raman scattering measurements on silver island films was also demonstrated.